Carbyne Complexes Research Articles

Reversible Transformation between Alkylidene, Alkylidyne, and Vinylidene Ligands in High-Valent Bis(phenolate) Tungsten Complexes

A tungsten alkylidyne complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(≡CEt)X] (X = OtBu, 1a) with a bridged bis(phenolate) ligand was prepared from the alkoxy precursor (tBuO)3W≡CEt and the corresponding bis(phenol). The tert-butoxy ligand in 1a was substituted against chloride or trimethylsilylmethyl to give 1b (X = Cl) or 1c (X = CH2SiMe3). X-ray diffraction studies of 1a and 1c showed an octahedral cis α-configuration of the metal atom. Hydride abstraction from the β-position of the n-propylidyne ligand in 1a and 1c with [Ph3C][B(C6F5)4] gave high-valent cationic vinylidene complexes [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(═C═CHMe)X][B(C6F5)4] (2a: X = OtBu; 2c: X = CH2SiMe3), while protonation of 1a with [PhNMe2H][B(C6F5)4] gave the cationic n-propylidene complex [{(OC6H2tBu2-4,6)2(SCH2CH2S)}W(═CHEt)(OtBu)][B(C6F5)4] (3a). The alkylidyne complex 1a was regenerated from the alkylidene 3a by deprotonation with Ph3P═CH2, whereas 1c was regenerated from 2c by nucleophilic attack by a hydride at the β-carbon of the vinylide...

Cleavage of carbon suboxide to give ketenylidene and carbyne ligands at a reactive tungsten site: a theoretical mechanistic study

The reaction of (MePPh2)4WCl2 with C3O2 results in stepwise cleavage of the two CC double bonds in C3O2 to give tungsten complexes containing phosphinoketenylidene and phosphinocarbyne ligands. The mechanism of this has been elucidated using density functional theory.

Dual nucleophilic substitution at a W(ii) η(2)-coordinated diiodo acetylene leading to an amidinium carbyne complex.

The synthesis and reactivity of a W(ii) C2I2 complex towards various nucleophiles are described. Soft, aprotic nucleophiles like 4-dimethylaminopyridine (DMAP) lead to substitution of one CO at tungsten, whereas reaction with an excess of benzylamine results in a dual nucleophilic substitution at the alkyne moiety involving the rearrangement to a novel cationic amidinium carbyne complex.

Molecular models of site-isolated cobalt, rhodium, and iridium catalysts supported on zeolites: Ligand bond dissociation energies

The chemistry of zeolite-supported site-isolated cobalt, rhodium, and iridium complexes that are essentially molecular was investigated with density functional theory (DFT) and the results compared with experimentally determined spectra characterizing rhodium and iridium species formed by the reactions of Rh(C2H4)2(acac) and Ir(C2H4)2(acac) (acac=acetylacetonate) with acidic zeolites such as dealuminated HY zeolite. The experimental results characterize ligand exchange reactions and catalytic reactions of adsorbed ligands, including olefin hydrogenation and dimerization. Two molecular models were used to characterize various binding sites of the metal complexes in the zeolites, and the agreement between experimental and calculated infrared frequencies and metal–ligand distances determined by extended X-ray absorption fine structure spectroscopy was generally very good. The calculated structures and energies indicate a metal–support-oxygen (M(I)O) coordination number of two for most of the supported complexes and a value of three when the ligands include the radicals C2H5 or H. The results characterizing various isomers of the supported metal complexes incorporating hydrocarbon ligands indicate that some carbene and carbyne ligands could form. Ligand bond dissociation energies (LDEs) are reported to explain the observed reactivity trends. The experimental observations of a stronger MCO bond than M(C2H4) bond for both Ir and Rh match the calculated LDEs, which show that the single-ligand LDEs of the mono and dual-ligand complexes for CO are ∼12 and ∼15kcal/mol higher in energy (when the metal is Rh) and ∼17 and ∼20kcal/mol higher (when the metal is Ir) than the single-ligand LDEs of the mono and dual ligand complexes for C2H4, respectively. The results provide a foundation for the prediction of the catalytic properties of numerous supported metal complexes, as summarized in detail here.

An anionic molybdenum amidato bisalkyl alkylidyne complex

The anionic high oxidation state molybdenum (VI) amidato bisalkyl alkylidyne complex [(Mo(N-2,6-Me2-C6H3)(CH2CMe3)2(CCMe2Ph)(Mg.Et2O-μ-Cl)2], 1, is reported. It forms via reaction of Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(O3SCF3)2.DME (DME = 1,2-dimethoxyethane), with an excess of neopentylmagnesium chloride. DFT calculations suggest a mesomerization between an amidato-alkylidyne and imido alkylidenato complex.

Heterometallic clusters derived from the unsaturated carbyne-bridged dimolybdenum complexes [Mo2(η5-C5H5)2(μ-CPh) (μ-PCy2) (CO)x] (x = 1, 2)

The carbonyl-bridged complex [Mo2Cp2(μ-CPh) (μ-PCy2) (μ-CO)] (1) reacted with [Fe2(CO)9] at room temperature to give the 46-electron trinuclear cluster [FeMo2Cp2(μ3-CPh) (μ-PCy2) (CO)5] (Mo–Mo = 2.6782(4) Å), and a similar Mo2Ru cluster was obtained upon reaction with [Ru3(CO)12] under irradiation with visible-UV light (Cp = η5-C5H5). Compound 1 reacted with [Co2(CO)8] at room temperature to give the 60-electron tetrahedral cluster [Co2Mo2Cp2(μ3-CPh) (μ-PCy2) (CO)7], which in solution exits as an equilibrium mixture of two isomers and presumably displays phosphide and carbyne ligands in a cisoid arrangement. This compound evolved thermally to give a third isomer having these ligands arranged in a transoid way (P–Mo–C = 126.3(1)o, Mo–Mo = 2.9612(6) Å). The dicarbonyl complex [Mo2Cp2(μ-CPh) (μ-PCy2) (CO)2] (2) reacted with W(CO)6 under visible-UV light irradiation to give two thermally unstable isomers of the 46-electron trinuclear cluster [Mo2WCp2(μ3-CPh) (μ-PCy2) (CO)6]. Reaction of 2 with [AuCl(PR3)] (R = Me, p-tol, iPr) in the presence of TlPF6 gave first the corresponding cationic clusters trans-[AuMo2Cp2(μ3-CPh) (μ-PCy2) (CO)2(PR3)]PF6, which then evolved thermally to the more stable isomers cis-[AuMo2Cp2(μ3-CPh) (μ-PCy2) (CO)2(PR3)]PF6 (Mo–Mo = 2.810(1) Å for R = iPr), selectively formed with a syn conformation of the carbyne and Cp ligands, except for the PMe3 complex. In contrast, reaction of 2 with CuCl led to a cluster of composition [CuMo2ClCp2(μ3-CPh) (μ-PCy2) (CO)2], presumably present as a monomer in solution, but certainly appearing in the solid state as a centrosymmetric dimer held by bridging Cu–Cl–Cu interactions (Mo–Mo = 2.8004(5), Cu–Cl = 2.309(1), 2.409(1) Å).

Phosphinoalkylidene and -alkylidyne Complexes of Titanium: Intermolecular C-H Bond Activation and Dehydrogenation Reactions.

The ethylene complex (PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu) or (PNP)Ti═CH(t)Bu(CH2(t)Bu) (PNP(-) = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ(2)-phosphinoalkylidene (PNP)Ti═CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti≡CPPh2] (C). By treatment of (PNP)Ti═CH(t)Bu(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)Ti═CDPPh2(C6D5) (1-d6) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12. Addition of MeOTf to 1 results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)Ti≡CPPh2Me(OTf) (2). Theoretical studies of 2 suggest a resonance structure having dominant Ti-C triple-bond character with some contribution also from a C-P multiple bond.

Concise Total Synthesis of Ivorenolide B.

An expeditious route to the potential immunosuppressive lead compound ivorenolide B (1) is described, which relies on the formation of the distinctive 1,3-diyne subunit embedded into the 17-membered framework of this target by ring-closing alkyne metathesis (RCAM). This key transformation was accomplished with the aid of the molybdenum alkylidyne complex 7, which turned out to be compatible with the acid sensitive propargylic alcohol substituents as well as the terminal alkyne unit present in the cyclization precursor. As the presence of such functionality had been detrimental for alkyne metathesis until very recently, this example illustrates the excellent application profile of this new catalyst as well as the rapidly increasing scope of the transformation. Its structural outreach can be further increased by subjecting cyclo-1,3-diynes to appropriate post-metathetic transformations, most notably with the help of alkynophilic gold or palladium catalysts. This aspect is illustrated by the conversion of the model compound 4 into various cyclophane products.

New Alkylidyne Complexes Featuring a Flexible Trianionic ONO3– Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions

Metal–carbon multiple bonds exhibit enhanced nucleophilicity at the α carbon within ONO trianionic pincer alkylidyne complexes. Defined as the inorganic enamine effect, this phenomenon is a result of the overlap of the N atom lone pair within the ONO3– ligand and a π bond from the metal–carbon multiple bond. Treating the proligand [OCH2NCH2O]H3 (2) with (tBuO)3W≡CR (where R = Et, tBu) results in the formation of the dianionic pincer complexes [OCH2NHCH2O]W≡CR(OtBu) (where R = Et (3-Et), tBu (3-tBuanti)). Deprotonation of 3-tBuanti by treatment with Ph3P═CH2 forms the anionic alkylidyne complex {[OCH2NCH2O]W≡CtBu(OtBu)}{CH3PPh3} (4-tBu). DFT calculations modeling 4-tBu reveal overlap of the N atom lone pair with a W≡C π bond. However, 4-tBu reacts with electrophiles preferentially at the pincer N atom as opposed to the W≡Cα group. Multinuclear NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes 3-Et, 3-tBuanti, and 4-tBu.

Five‐Membered Cyclic Metal Carbyne: Synthesis of Osmapentalynes by the Reactions of Osmapentalene with Allene, Alkyne, and Alkene

AbstractThe synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five‐membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five‐membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient.

Five-membered cyclic metal carbyne: synthesis of osmapentalynes by the reactions of osmapentalene with allene, alkyne, and alkene.

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient.

Cycloaddition and C–H Activation Reactions of a Tantalum Alkylidyne

The alkylidyne complex (C5Me5)Ta(≡CPh)(PMe3)2Cl (1) was first reported by Schrock in 1978, but little if any follow-up work on 1 or other group 5 metal alkylidynes has been reported. This work discloses two avenues of reactivity of 1. Treatment of 1 with 3-hexyne resulted in the formation of a tantalacyclobutadiene. Abstraction of chloride from 1 led to a mixture of products that included isomers of [(C5Me5)Ta(═CHPh)(CH2PMe2)(PMe3)]+ (4), in which a C–H bond of a PMe3 ligand was added across the Ta≡C bond. The C–H activation was found to be reversible, and the equilibrium mixture functioned as an equivalent of a cationic Ta alkylidyne in reaction with 3-hexyne, producing a cationic tantalacyclobutadiene (6). Compounds 4 and 6 were structurally characterized in the solid state by XRD methods, with 6 being the first structurally characterized metallacyclobutadiene in group 5.

Silica-Supported Tungsten Carbynes (≡SiO)xW(≡CH)(Me)y (x = 1, y = 2; x = 2, y = 1): New Efficient Catalysts for Alkyne Cyclotrimerization

The activity of silica-supported tungsten carbyne complexes (≡SiO)xW(≡CH)(Me)y (x = 1, y = 2; x = 2, y = 1) toward alkynes is reported. We found that they are efficient precatalysts for terminal alkyne cyclotrimerization with high TONs. We also demonstrate that this catalyst species is active for alkyne cyclotrimerization without the formation of significant alkyne metathesis products. Additional DFT calculations highlight the importance of the W coordination sphere in supporting this experimental behavior.

Benzylidyne Bridges from Diphenylacetylene and a Methylidyne Bridge from Methylmagnesium Chloride

Preparation of the dimeric starting compound [(C5H2tBu3-1,2,4)Fe(μ-Br)]2 (1) gave a small amount of its green sodium bromide adduct [(C5H2tBu3-1,2,4)Fe(μ-Br)2Na(DME)2] (2). Reduction of 1 under argon in the presence of diphenylacetylene yielded dark red [(C5H2tBu3-1,2,4)Fe(μ-CPh)]2 (3) containing two bridging benzylidyne ligands resulting from diphenylacetylene scission. The dicobalt methylidyne complex [{(C5HiPr4)Co}2(μ-CH)(μ-H)] (5) was obtained in high yield from the reaction of [(C5HiPr4)Co(μ-Cl)]2 (4) with two equivalents of methylmagnesium chloride. According to DFT calculations the dinuclear alkylidyne complexes 3 and 5 contain d6 metal centers, namely, iron(II) (3) and cobalt(III) (5). Whereas the benzylidyne ligands of the iron complex 3 display an empty orbital of pz character at the bridging carbon atom, the pz orbital of the methylidyne carbon in the dicobalt complex 5 is filled with a lone pair of electrons.

A new ONO(3-) trianionic pincer ligand with intermediate flexibility and its tungsten alkylidene and alkylidyne complexes.

This report details the synthesis and characterization of the semi-flexible [ON(CH2)O]H3 (1) ligand and its W(vi)-alkylidene and alkylidyne complexes. The alkylidyne complex [ONH(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu) (2) forms as a result of alcoholysis of 1 with ((t)BuO)3W[triple bond, length as m-dash]C(t)Bu. Complex 2 evolves to [ON(CH2)O]W[double bond, length as m-dash]CH(t)Bu(O(t)Bu) (3) through proton migration from the N atom of the pincer ligand to the W[triple bond, length as m-dash]Cα bond. Deprotonation of 2 or 3 with Ph3PCH2 affords the anionic alkylidyne {CH3PPh3}{[ON(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu)} (4). Complex 4 exhibits pincer-ligand-centered reactivity with electrophiles (H(+), Me(+), and TMS(+)), in spite of its enhanced inorganic enamine interaction. Addition of 2 equiv. of HCl to 4 yields the W(vi)-neopentyl complex [ON(CH2)O]W(CH2(t)Bu)(O(t)Bu)(Cl) (5). MeOTf or TMSOTf addition to 4 generates the dianionic pincer ligated alkylidynes [ONR(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu) (R = Me (6-Me); TMS (6-TMS)). Complexes 2-5 were characterized by multinuclear NMR spectroscopy, and combustion analysis. Complexes 4 and 5 were also characterized by single crystal X-ray diffraction. This work bridges the gap in the series involving W(vi)-alkylidynes ligated to the rigid [CF3-ONO](3-), and the flexible [O(CH2)N(CH2)O](3-) ligands. DFT computations permit comparison of the inorganic enamine effect within alkylidynes supported by all three trianionic-pincer type ONO ligands.

Simple addition of silica to an alkane solution of a Wilkinson WMe6 or Schrock W alkylidyne complex gives an active complex for saturated and unsaturated hydrocarbon metathesis.

The addition of partially dehydroxylated silica (PDS) to a solution of the Wilkinson WMe6 as well as the Schrock W neopentylidyne tris neopentyl complex catalyzes linear or cyclic alkanes to produce respectively a distribution of linear alkanes from methane up to triacontane or a mixture of cyclic and macrocyclic hydrocarbons. This single catalytic system transforms also linear α-olefins into higher and lower homologues via an isomerization/metathesis mechanism (ISOMET). This complex is also efficient towards functionalized olefins. Unsaturated fatty acid esters (FAEs) are converted into diesters corresponding to self-metathesis products.

ChemInform Abstract: A Six‐Coordinated Cationic Ruthenium Carbyne Complex with Liable Pyridine Ligands: Synthesis, Structure, Catalytic Investigation, and DFT Study on Initiation Mechanism.

Abstract A novel six-coordinated high-valence cationic ruthenium carbyne complex bearing two liable pyridine ligands was prepared in high yield by the reaction of the ruthenium-based complex (IMesH2)(Cl)2(C5H5N)2Ru CHPh [IMesH2=1,3-dimesityl-4,5-dihydroimida-zol-2-ylidene] with excess iodine as an oxidant in CH2Cl2 at 25 °C under N2. The new ruthenium carbyne-based complex shows moderate to good catalytic activity for ring-closing metathesis reactions. Importantly, no double bond isomerization by-product was produced at elevated reaction temperatures (100 °C–137 °C) in the reaction catalyzed by the synthesized ruthenium carbyne complex. A mechanism involving the in situ conversion of the ruthenium carbyne through the addition of an iodide to the carbyne carbon was also proposed, and DFT calculations were performed to explain the initiating mechanism.

Mild P–P Bond Cleavage in the Methyldiphosphenyl Complex [Mo2Cp2(μ-PCy2)(μ-κ2:κ2-P2Me)(CO)2] To Give Novel Phosphide-Bridged Trinuclear Derivatives

Reactions of the title diphosphenyl complex with [Fe2(CO)9] and [W(CO)4(THF)2] gave the trinuclear species [Mo2FeCp2(μ3-P)(μ-PCy2)(μ3-PMe)(CO)5] and [Mo2WCp2(μ3-P)(μ-PCy2)(μ3-PMe)(CO)6] following from formal insertion of the 14-electron fragments Fe(CO)3 and W(CO)4, respectively, in the P-P bond of the diphosphenyl ligand and formation of a new heterometallic bond [Mo-Fe = 2.9294(6) Å and Mo-W = 3.146(1) Å]. Reactions of the diphosphenyl complex with the tetrahydrofuran adducts [MLn(THF)] (MLn = MnCp'(CO)2, W(CO)5) led instead to trinuclear diphosphenyl complexes [Mo2MCp2(μ-PCy2)(μ3-κ(2):κ(2):κ(1)-P2Me)(CO)2Ln] following from coordination in each case of the corresponding 16-electron fragment MLn to the lone-pair-bearing P atom of the P2Me ligand. However, these diphosphenyl complexes were unstable and decomposed at room temperature or under mild heating by the release of methylphosphinidene (PMe), to give the corresponding derivatives [Mo2MCp2(μ3-P)(μ-PCy2)(CO)2Ln] displaying trigonal-planar phosphide ligands, giving rise to strongly deshielded (31)P NMR resonances (δP ca. 1100 ppm), while being involved in strong π bonding with the unsaturated Mo2 center of these molecules [Mo-Mo = 2.749(1) Å and Mo-P = ca. 2.30 Å when M = W]. An isolobal analogy could be established between the P→MLn fragments in these products and a carbyne ligand (CR), supported by density functional theory calculations on the tungsten compound, which also enabled an easy interpretation and prediction of their chemical behavior. Thus, the manganese complex could be reversibly carbonylated (pCO = ca. 3 atm, 293 K) to give the corresponding electron-precise pentacarbonyl [MnMo2Cp2Cp'(μ3-P)(μ-PCy2)(CO)5] [Mo-Mo = 3.1318(7) Å], a process also involving a trans-to-cis rearrangement of the Mo2Cp2 subunit. On the other hand, decarbonylation of the tungsten complex was accomplished in a refluxing toluene solution to give the hexacarbonyl [Mo2WCp2(μ3-P)(μ-PCy2)(μ-CO)(CO)5], a derivative containing an unsaturated 30-electron dimolybdenum center with an intermetallic triple bond.

A six-coordinated cationic ruthenium carbyne complex with liable pyridine ligands: synthesis, structure, catalytic investigation, and DFT study on initiation mechanism

A novel six-coordinated high-valence cationic ruthenium carbyne complex bearing two liable pyridine ligands was prepared in high yield by the reaction of the ruthenium-based complex (IMesH2)(Cl)2(C5H5N)2RuCHPh [IMesH2=1,3-dimesityl-4,5-dihydroimida-zol-2-ylidene] with excess iodine as an oxidant in CH2Cl2 at 25 °C under N2. The new ruthenium carbyne-based complex shows moderate to good catalytic activity for ring-closing metathesis reactions. Importantly, no double bond isomerization by-product was produced at elevated reaction temperatures (100 °C–137 °C) in the reaction catalyzed by the synthesized ruthenium carbyne complex. A mechanism involving the in situ conversion of the ruthenium carbyne through the addition of an iodide to the carbyne carbon was also proposed, and DFT calculations were performed to explain the initiating mechanism.

Total Synthesis of the Biphenyl Alkaloid (−)‐Lythranidine

AbstractA sequence comprising a ring‐closing alkyne metathesis of a propargyl alcohol derivative, followed by a ruthenium‐catalyzed redox isomerization of the derived cycloalkyne and a transannular aza‐Michael addition allowed the formation of the distinguishing piperidine‐metacyclophane framework of the Lythraceum alkaloid lythanidine in a few high‐yielding steps. This application attests to the excellent functional‐group tolerance of a molybdenum alkylidyne complex endowed with triphenylsilanolate ligands, which enabled the macrocyclization even in the presence of protic functionalities, and thus illustrates the power of contemporary catalytic acetylene chemistry for target‐oriented synthesis.